Ok, I tested this out and without the trailing slash on the core_path you end up with something like this:
$lang_files = Array
(
    [ext/foo/bar/language/en/permissions_foo.php] => foo/bar
    [language/enacp/permissions_phpbb.php] => /
    [language/enmods/permissions_mod.php] => /
)
Notice /enacp/, /enmods/ instead of /en/acp/, /en/mods/ respectively.

The finder sanitizes the 'extension_directory' and removes any trailing slashes and therefore it is not necessary. However, it seems that the '/' at the beginning of the extension_directory is unnecessary. It didn't seem to matter when I set the extension directory as 'language/...' or '/language/..'. Should I remove the beginning slash?